The present invention relates to testing of radio frequency (RF) signal transmitters designed to perform beamforming, and in particular, to assessing receiver signal reception performance during wireless beam steering operation of a radio frequency (RF) data packet signal transceiver capable of multiple user, multiple input, multiple output (MU-MIMO) operation.
Many of today's electronic devices use wireless signal technologies for both connectivity and communications purposes. Because wireless devices transmit and receive electromagnetic energy, and because two or more wireless devices have the potential of interfering with the operations of one another by virtue of their signal frequencies and power spectral densities, these devices and their wireless signal technologies must adhere to various wireless signal technology standard specifications.
When designing such wireless devices, engineers take extra care to ensure that such devices will meet or exceed each of their included wireless signal technology prescribed standard-based specifications. Furthermore, when these devices are later being manufactured in quantity, they are tested to ensure that manufacturing defects will not cause improper operation, including their adherence to the included wireless signal technology standard-based specifications.
Testing of such wireless devices typically involves testing of the receiving and transmitting subsystems of the device under test (DUT). The testing system will send a prescribed sequence of test data packet signals to a DUT, e.g., using different frequencies, power levels, and/or signal modulation techniques to determine if the DUT receiving subsystem is operating properly. Similarly, the DUT will send test data packet signals at a variety of frequencies, power levels, and/or modulation techniques for reception and processing by the testing system to determine if the DUT transmitting subsystem is operating properly.
For testing these devices following their manufacture and assembly, current wireless device test systems typically employ testing systems having various subsystems for providing test signals to each device under test (DUT) and analyzing signals received from each DUT. Some systems (often referred to as “testers”) include at least a vector signal generator (VSG) for providing the source signals to be transmitted to the DUT, and a vector signal analyzer (VSA) for analyzing signals produced by the DUT. The production of test signals by the VSG and signal analysis performed by the VSA are generally programmable (e.g., through use of an internal programmable controller or an external programmable controller such as a personal computer) so as to allow each to be used for testing a variety of devices for adherence to a variety of wireless signal technology standards with differing frequency ranges, bandwidths and signal modulation characteristics.
Wireless devices, such as cellphones, smartphones, tablets, etc., make use of standards-based technologies, such as IEEE 802.11a/b/g/n/ac/ad/ax/ay (“Wi-Fi”), 3GPP LTE, and Bluetooth. The standards that underlie these technologies are designed to provide reliable wireless connectivity and/or communications. The standards prescribe physical and higher-level specifications generally designed to be energy-efficient and to minimize interference among devices using the same or other technologies that are adjacent to or share the wireless spectrum.
Tests prescribed by these standards are meant to ensure that such devices are designed to conform to the standard-prescribed specifications, and that manufactured devices continue to conform to those prescribed specifications. Most devices are transceivers, containing at least one or more receivers and transmitters. Thus, the tests are intended to confirm whether the receivers and transmitters both conform. Tests of the receiver or receivers (RX tests) of a DUT typically involve a test system (tester) sending test packets to the receiver(s) and some way of determining how the DUT receiver(s) respond to those test packets. Transmitters of a DUT are tested by having them send packets to the test system, which then evaluates the physical characteristics of the signals sent by the DUT.
Referring to FIGS. 1A and 1B, more recent versions of the IEEE 802.11 standards provide for beam steering, or beamforming, to enable transmission and reception of more spatially directional signal streams having higher effective signal-to-noise ratios (SNRs) and higher data rates. Also enabled are communications with and/or between devices having multiple inputs and/or multiple output signal streams in either single user (SU) mode (FIG. 1A), in which the multiple signal streams 3 of the source (“beamformer”) 1 are steered to all input ports of a client (“beamformee”) 2, or multiple user (MU) mode (FIG. 1B), in which subsets 3a, 3b of the signal streams of the source 1 are steered to the input ports of respective clients 2a, 2b. 
More particularly, the IEEE 802.11ac standard provides specifications for multiple user, multiple input, multiple output (MU-MIMO) operations. The “MIMO” capability is the use of multiple antennas at the receiver (multiple input or “MI . . . ”) and the transmitter (multiple input or “MO . . . ”) to improve communication performance through advanced digital signal processing. It takes advantage of the separate transmit/receive chains associated with each antenna improve the link robustness and/or increase the data rate. This enables wireless communications having higher signal bandwidths to enable higher data throughputs. The “MU” capability allows multiple devices to communicate separately, e.g., using a single access point (AP). In other words, as opposed to single-user MIMO (SU-MIMO) operation where two devices communicate only with each other via all available antennas, MU-MIMO allows a terminal to transmit and receive signals to and from multiple users in the same frequency band simultaneously.
Referring to FIG. 2, during MU-MIMO operation between a source 10 (e.g., an AP with four antennas) and multiple clients 12 (e.g., a laptop computer with dual antennas), 14 (a smartphone with a single antenna), 16 (a cellular telephone with a single antenna), a steering matrix is used to inform the source 10 about received signals. The station (client) provides beamforming feedback by generating and providing a beamforming feedback matrix (BFM), e.g., by using the received signal header to generate a compressed beamforming feedback (CBF) matrix in response to a sounding packet (SP), which in this example could be a Null Data Packet (NDP) packet. The CBF matrix (V-matrix) is then transmitted back to the source as part of the response packet generated as a response to the NDP packet.
Accuracy of the CBF matrix plays a significant role in determining how effective the MU-MIMO steering operations are. To get the best possible beamforming feedback matrix accuracy, one needs a very clean input signal to the client receiver, and any noise and/or distortion added to the signal by the receiver must be minimized. Quality of signal emissions by the client transmitter may be fairly simple to measure using a good VSA (e.g., so long as a common local oscillator is used to maintain reference signal phase coherency), but added effects on signal quality by client receiver contributions are more complex to determine. One solution would be to measure the receive error vector magnitude (EVM). However, access to digitized IQ data samples is typically not possible in many testing environments, especially during manufacturing tests.
Referring to FIG. 3, alternatively, receive quality is often measured using packet error (PER) testing. However, once the receiver reaches a certain EVM level, further improvement in the EVM has minimal effect on the PER. Therefore, even testing for sensitivity of the receiver will not reveal the true receiver EVM (even using a high quality VSG), since accuracy and statistical variations of the output power of a VSG are worse than variations in PER due to degraded EVM. Hence, traditional PER cannot be effectively used to test for receiver quality of the receiver, but only for whether it is sufficient to pass a PER requirement. For example, IEEE standards specify an EVM of −35 dB for transmit signal (TX) quality, but it is believed that −41 dB is optimal for MU-MIMO. Thus, a receive signal (RX) EVM of −35 dB will yield essentially the same PER as a RX EVM of −41 dB (with less than 0.5 dB variation). Accordingly, a better way to test the quality of a MU-MIMO beamforming receiver is needed.